Lead is known to be a toxic substance that interferes with the production of red blood cells in the human body. It has been determined that lead prevents the body from putting iron into hemoglobin in the red cell generation process occurring in the bone marrow. This lack of iron in the red blood cells permits zinc to take the place of the missing iron in the hemoglobin. Consequently the amount of zinc protoporphyrin, a product of the generation of hemoglobin, is a measure of lead poisoning of the body where the lack of iron in the red blood cells results from the injection of lead.
The presence of zinc protoporphyrin in the blood can be due to causes other than lead poisoning. For example, if the body has an insufficient intake or iron due to a poor or unbalanced diet, the body will be unable to supply the iron needed in the production of red blood cells and zinc will tend to take the place of the missing iron in the cells. As another example, where profuse or continual bleeding necessitates the replacement of lost blood, the production of new red blood cells may require more iron than the body has in reserve. Whatever the cause, the presence of zinc protoporphyrin indicates a malfunction in the generation of hemoglobin and a large amount of zinc protoporphyrin usually is a sign that medical treatment is needed.
Zinc protoporphyrin is a fluorescent substance and when exited by radiation in the band from about 415 nanometers (nm.) to 440 nm. in wavelength, fluoresces and emits light whose wavelength is in the range from 570 nm. to 660 nm. The peak emission of fluorescent light from zinc protoporphyrin occurs when the excitation radiation is at 424 nm. and when so excited the maximum fluorescent intensity occurs at 595 nm.
It has been proposed to measure the quantity of zinc protoporphyrin in a blood sample by fluorometry to provide a quick and easy method for detecting lead poisoning and obtaining a measure of its level in the body. However, the amount of zinc protoporphyrin in whole blood is never very large and because of its low concentration, its fluorescent intensity in a blood sample is very low. The amount of radiation emitted by zinc protoporphyrin is directly proportional to its concentration in the blood.
In the preferred mode of use of the invention for the detection of lead poisoning, fluorescence analysis is carried out directly without dilution of the blood sample and therefore the sample is excited by front face excitation illumination. For accurate results, the "front face" technique requires the sample to absorb all the excitation radiation. Further, by employing an excitation source that has minimal infrared and ultraviolet emission, chemical decomposition of the blood sample by the action of the radiant energy is largely avoided. Although not an essential feature of the invention, photolysis can be further inhibited by turning off the excitation source immediately after fluorometric analysis or dropping a mechanical shutter.